Acoustic resonator structures such as surface acoustic wave (SAW) devices 10 use one or more interdigitated transducers (IDTs) 111 provided on a piezoelectric substrate 100 to convert electrical signals to acoustic waves and vice versa, as schematically illustrated in FIG. 1A. Such SAW devices or resonators are often used in filtering applications. Radio frequency (RF) SAW technology has excellent performances, such as high isolation and low insertion losses, and are widely used for RF duplexers (DPXs) in wireless communication applications. In order to be competitive over RF DPXs based on RF bulk acoustic wave (BAW) technology, the device performance of RF SAW devices has to be improved and, especially, the temperature stability of the frequency response is demanded.
The temperature dependence of the operating frequency of SAW devices, or the thermal coefficient of frequency (TCF), is not only dependent on the changes of the spacing, illustrated in FIG. 1A as spacing S, between the interdigitated fingers of the IDTs, which are generally due to the relatively high coefficient of thermal expansion (TCE) of the commonly used piezoelectric substrates, but depend also on the thermal coefficient of velocity (TCV), as an expansion or contraction of the piezoelectric substrate that comes along with an increase or decrease of the SAW velocity.
The recently published article by K. Hashimoto, M. Kadota, et al., “Recent Development of Temperature Compensated SAW Devices,” IEEE Ultrason. Symp. 2011, pages 79-86, gives an overview of current approaches used to overcome the problem of temperature dependence of the frequency response of SAW devices, in particular, the approach of the SiO2 overlay.
As shown schematically in FIG. 1B, the latter approach concerning the SiO2 overlay comprises the step S12 of metalizing the piezoelectric substrate 100, leading to metalized parts 110, and the subsequent formation step S10 of a dielectric layer 120, in particular, an SiO2 layer, onto the entire surface of the piezoelectric substrate 100 and the metalized parts 110. A further planarization step S13 may be performed, depending on whether one envisaged a SAW device with a convex top surface 101 or a SAW device with a flat top surface 102 as the final device. However, this approach is rather limited for several reasons. The choice of materials used for metalized parts 110 and the choice of deposition techniques used for these materials are restricted due to the compatibility requirement with the piezoelectric substrate 100, aiming for a good electrical (ohmic) contact. Further, the choice of materials used for the dielectric layer 120 covering the metalized parts 110 and the piezoelectric substrate 100, and the choice of deposition techniques used for that dielectric layer 120 are restricted because the thermal budget employed during the formation of the dielectric layer 120 has to be compatible with the material used for metalized parts 110 and the piezoelectric substrate 100 in order to avoid degradation of the piezoelectric properties, the degradation of the electrical properties of the metalized parts 110, or avoid diffusion of metal to either the piezoelectric substrate 100 or the dielectric layer 120 formed on top of the metalized structure. In addition, the rather high TCE of the commonly used piezoelectric substrates 100 can further cause manufacturing problems due to warping or bending or induced strain if several materials with different TCE are in contact with each other and the temperatures applied exceed the maximal tolerated limit, as may be the case for the metallization 110 formed on top of the piezoelectric substrate 100, and the thermal budget employed during the formation of the dielectric layer 120 may lead to delamination of the metalized parts 110 or even breakage of the wafer. Further, the formation of the dielectric layer 120 at rather low temperatures, as can be the case for certain amorphous SiO2 layers, leads to rather low quality material with reduced acoustic properties and, thus, limits the performance of SAW technology based on it. Even further, unavoidable growth defects due to the deposition of the dielectric layer 120 on both the metalized parts 110 and the piezoelectric substrate 100, together with the fact that the metalized parts 110 comprise horizontal and vertical parts with respect to the growth direction of the dielectric layer 120, lead to parasitic electro-acoustic effects, especially at the intersections of the vertical and horizontal parts and, thus, losses of performance of the SAW device 10.